1. Field of the Invention
The present invention relates to a reversible electrode used for electrochemical devices such as a battery, an electrochromic display, a sensor, and a memory.
2. Description of the Related Art
An electrode material used for a lightweight battery with a high energy density, an electrochromic display with a large area, or a biochemical sensor using a microelectrode is required to be lightweight, to have a high theoretical energy density, and to be excellent in durability.
A conducting polymer electrode has extensively been studied since Shirakawa et al. discovered conducting polyacetylene in 1971. This is because the use of a conducting polymer enables a battery which is lightweight and has a high energy density. Polyacetylene is, however, impractical in use due to its chemical instability with respect to moisture and oxygen in the air. Subsequently, since n-electron conjugated conducting polymers, which are chemically stable, such as polyaniline, polypyrrol, and polythiophene were discovered, a lithium secondary battery and the like using these electrode materials as a cathode have been developed. In addition to these conducting polymers, some electrode materials which are lightweight and have a high energy density, such as sulfur and a compound containing sulfur have been studied.
An electrode using a conducting polymer as a cathode incorporates anions in an electrolyte as well as cations when subjected to an electrode reaction. Thus, the electrolyte acts as a medium for moving ions and participates in the cell reaction simultaneously as a cathode active material. Because of this, as the electrode reaction proceeds, the electrolyte decreases in amount, requiring that an electrolyte in an amount enough to compensate for a battery's capacity should be supplied in a battery. As a result, an energy density of the battery is decreased. A lithium secondary battery using a conducting polymer as a cathode has an energy density of 20 to 50 Wh/kg, i.e., about 1/2 of that of a usual secondary battery such as a nickel-cadmium battery or a lead storage battery. Moreover, for example, a sodium-sulfur battery using sulfur as a cathode is slow to perform an electrode reaction at room temperature, so that this type of battery is used only under a high temperature condition where the electrode reaction proceeds relatively rapidly. Japanese Laid-Open Patent Publication No. 3-93169 discloses a solid electrochemical battery which recharges itself, using an organic sulfur compound such as dithiouracil as a cathode. This solid electrochemical battery does not have a sufficiently high electrode reaction rate at room temperature.